The correct way is "use 5.008". (You probably don't want to "use 5.008001", as that will also kill the Makefile.PL on perl 5.8.0.)

I don't know for a fact that CPAN.pm will handle that correctly, but I would be surprised if it didn't - and if you find that it doesn't handle it correctly then a bug report is in order.

If you have perl 5.6 and you want to check, try installing Net-SSH2-0.09 (whose Makefile.PL does "use 5.008") using CPAN.pm.

Of course, if there once was a version of CPAN.pm that didn't do the right thing, then it's quite possible that some people are still using that version ... probably not much you can do about that. (I expect that the cpan testers would be using an uptodate and relatively bug-free version of CPAN.pm.)

Cheers,
Rob